1. Field of the Invention
The invention relates to an optical mechanism, and in particular to an optical mechanism for precisely controlling the angle of an incident light beam within a large incident angle range.
2. Description of the Related Art
Currently, an optical measuring apparatus usually needs an optical mechanism system that can specify a single detecting point under any circumstances when measuring a specific position. For example, to prevent design troubles for an ellipsometer during the above-stated measurement, one method is that an incident angle is fixed to perform a multi-wave length measurement, and then the number of unknown quantities is decreased with the cooperation of an appropriate material model. However, this method completely depends on the accuracy of the dispersion formula of the material model, and therefore, it is theoretically inferior to changing the incident angle. It is noted that the principle of changing an incident angle is to measure the intensity or the phase of a reflected light beam by precisely controlling an incident angle or largely changing the incident angle. Then, a signal corresponding to a specific position can be obtained using an inverse mathematical calculation. Nevertheless, it is necessary to have a particular design on the optical mechanism system with a measured point fixed because the angle of the incident light beam is continuously changed.
Basically, the conventional incident angle changeable optical mechanism can be grouped into an arm-rotating type, a single lens focusing type 10 (referring to FIGS. 1(A) and 1(B)) and a multi-lens focusing type 20 (referring to FIGS. 2(A) and 2(B)) of optical mechanisms. However, these optical mechanisms have several defects, for example, the change of an incident angle is too small, and the spherical phase difference can not be compensated, resulting in more complicated optical mechanisms. Therefore, it is very difficult to find an optical mechanism which can precisely provide a large range of incident angle and make optical paths more closed.
As to the arm-rotating type optical mechanism, measuring a changeable incident angle is performed by using two rotating motors to drive two rotating arms called an incident arm and a reflecting arm, wherein a transmitting terminal is set on the incident arm while a receiving terminal is set on the reflecting arm. However, the optical head of this arm-rotating type optical mechanism must be divided into two parts on function and space because of the existence of the two rotating motors. This will cause a large volume. Meanwhile, it is difficult to increase the accuracy of this optical mechanism, resulting in an expensive cost.
With respect to the single lens focusing type incident angle changeable optical mechanism 10 as shown in FIGS. 1(A) and 1(B), it includes a lens 11, two fixed reflecting prisms 12, 13 and two moveable reflecting prisms 14, 15 which can be moved up and down by use of a single axis shift stage. When the reflecting prisms 14, 15 are moved to the top level of the reflecting prisms 12, 13, a corresponding incident angle is approximately 2.degree.. As shown in FIG. 1(A), when the reflecting prisms 14, 15 is moved to the bottom level of the reflecting prisms 12, 13, a corresponding incident angle is approximately 41.degree.. Since it is difficult to have the numerical aperture (NA) of a single lens more than 1, an incident angle can be hardly over 45.degree.. This results in that the incident angle is only approximately located in the range of 0-45.degree.. That is, an incident angle over 45.degree. can not be provided. Moreover, the single lens focusing type optical mechanism has a problem of spherical phase difference, so it will cause non-unique focuses.
As to the multi-lens focusing type incident angle changeable optical mechanism developed by modifying the single lens focusing type optical mechanism (as shown in FIGS. 2(A) and 2(B)), it includes lenses 21, 22, 23, two fixed reflecting prisms 22, 23 and two moveable reflecting prisms 24, 25 which can be moved up and down by use of a single axis shift stage. With this multi-lens focusing type incident angle changeable optical mechanism, an incident light beam and a reflected light beam can be still parallel to an original light beam after traveling a specific distance. As shown in FIG. 2(A), when the reflecting prisms 24, 25 is moved to the top level of the reflecting prisms 22, 23, a corresponding incident angle is approximately 2.degree.. As shown in FIG. 2(B), when the reflecting prisms 24, 25 is moved to the bottom level of the reflecting prisms 22, 23, a corresponding incident angle is approximately 88.degree.. The multi-lens focusing type incident angle changeable optical mechanism 20 not only can make optical paths more close to each other, but also can have a larger incident angle. However, it is very difficult for the conventional multi-lens focusing type incident angle changeable optical mechanism on design, assembly and calibration. As stated above, the general focusing lenses having a problem of spherical aberration will cause focuses non-unique. Furthermore, with such conventional incident angle changeable optical mechanisms, a fixed point can not be measured through a transparent or translucent dielectric.